1. Field of the Invention
The present invention relates generally to improvements in systems for storing computer files, and, more particularly, but not by way of limitation to systems including hard disk drives.
2. Brief Description of the Prior Art
Hard disc drives in which computer files are magnetically stored in surface coatings on rotating metal discs have increasingly become the choice of computer users for long term file storage. The advantages offered by such drives are a large storage capacity coupled with a short file access time. Thus, for example, drives capable of storing over a gigabyte of data with an average access time of the order of 10 milliseconds are now available.
With an ever increasing demand for rapid transfer of files between a host computer and a data storage system the possibility of parallel operation of plurality of hard disc drives, integrated into a single data storage system, has become increasingly attractive. For example, if lenqthy data words were to be divided into a number of bytes and transferred simultaneously between a host computer and an equal number of disc drives, the transfer rate for a file would effectively be multiplied by the number of bytes in the data words. Moreover, the use of a number of disc drives in an integrated system of disc drives, even without parallel operation, offers an economical way of achieving a large file storage capacity with little effect on data storage time.
While the concept of forming a number of hard disc drives into a data storage system is clearly desirable, the practicalities of doing so are another. Thus, where a plurality of disc drives are to be operated in parallel, the cost of carrying out such operation has been a drawback. For parallel operation to be feasible, the rotation of discs of the drives in the system must be synchronized so that each disc of each drive must pas through an orientation constituting an index for the reading and writing of files at the same time. While this synchronization can be achieved by providing the disc drives with appropriately programmed microcomputers, or complex custom circuits, dedicated to the task of controlling and synchronizing the operation of motors which rotate the discs, such approach increases costs of manufacture.
While the direct costs of using microcomputers dedicated solely to the task of motor speed operation or complex custom circuits for the same purpose, there are also hidden costs. As will be clear from the above, synchronization necessarily entails controlling the speeds of the motors of the disc drives of a data storage system in a phase mode of operation; that is, controlling the speeds to cause passage of the discs through the "index" orientation at a specific time. At times, phase control is neither necessary nor desirable; for example, in a lap top computer in which a disc drive will be subjected to rapid physical movements, it is preferable to operate the single disc drive to which the computer is limited by space considerations in the more robust frequency mode in which motor speed control is directed solely toward maintaining a constant disc rotation speed. Thus, unless a parallel capability can be inexpensively achieved, it would not be realistic to provide a single disc drive with both phase and frequency modes of operation. Thus, additional costs can be incurred in the separate design and manufacture of drives used for different purposes.